The Lark

We only see this "lark" (a bird) once but it's the most stunning and dominant image in the poem. It helps us to understand the speaker's changing mood and his dramatic spiritual transformation. Over the course of the sonnet, he goes from being in the depths of despair to a state of spiritual nirvana that's expressed as the blissful singing of a bird.

• Lines 10-12: Here, the speaker uses a simile comparing his once depressed mood to a "lark" that rises up from the "sullen earth" and sings "hymns" at heaven's gate. That's pretty dramatic, wouldn't you say? A "hymn" is a religious song praising God, so there's a suggestion here that our speaker feels closer to God than he has in the past. 
• The "lark" simile also reminds us that our speaker is in a completely different mind frame now than he was at the beginning of the sonnet. At line 3, he said that "heav'n" was "deaf" to his cries—meaning, God wasn't answering his prayers. But now, our speaker is no longer crying. Instead, he feels like a bird that's happily singing away at "heaven's gate." Is that because "heaven" (a.k.a. God) is no longer "deaf"? 
• And, what is it that's brought about this sudden and dramatic change in our speaker? He tells us at line 10 that he feels like a "lark" when he remembers the "sweet love" of another human being. That's interesting because it's NOT God that has brought about our speaker's spiritual transformation. It's the love of some unnamed mystery person that's got our speaker's spirits soaring. So, maybe our speaker doesn't feel any closer to God than he did before. Maybe he's just decided that he's found a new kind of "heaven."
• (Brain Snack: Shakespeare's kind of got a thing for larks. He inserts an entire poem about a lark singing at "heaven's gate" in a play called Cymbeline. And, he makes a really big deal about a lark in Romeo and Juliet, which you can learn more about here.